During air conditioner (a/c) operation in a heating, ventilating and air conditioning (HVAC) system in a vehicle, moisture condenses on the evaporator core from out of the air that is being conditioned. The air is cooled by an evaporator and moisture physically condenses on the evaporator core. Because of the core design, this moisture often accumulates on the core fins. Because the evaporator core must be an efficient heat exchanger, it is designed with louvered or cupped fins. While these fins are efficient for forced air convection, they naturally tend to retain moisture during and after normal a/c operation. A blower motor moves air across the fins at various selectable speeds depending upon the amount of heat exchange that is required. The amount of heat exchange depends upon the amount of cooling required for operator and passenger comfort. At higher blower speeds, there is more condensing action on the evaporator core but also more moisture likely to be blown off of the evaporator core.
When the vehicle is inoperative or the HVAC is turned off, air surrounds the moisture laden evaporator core within a plenum, chamber or ducting system. This provides an environment for bacteria, fungus and other organisms which can cause offensive odors in the vehicle. U.S. Pat. No. 3,662,818 which issued May 16, 1972 to Snyder addresses the moisture problem by providing a purge outlet which is open to the passenger compartment of the vehicle when the HVAC system is inoperative to maintain a continuous flow of air through the plenum chamber. This air flow prevents the formation of odor and humidity in the plenum chamber. Thereafter, when the system is activated, the purge outlet is closed. Unfortunately, there is air flow only when the ignition switch is on, therefore, moisture remains on the evaporator core to promote odor.
Later attempts to remove moisture operate the blower after the ignition switch is turned off for a set period of time regardless of moisture conditions which is inefficient. U.S. Pat. No. 5,385,028 which issued Jan. 31, 1995 to Gavlak improves moisture removal by briefly turning the blower on high speed to blow off excess moisture, then turning the blower on low speed or off while heating the core by recirculating refrigerant. Thereafter, when a high enough temperature for moisture elimination is reached, the blower and recirculation of refrigerant is discontinued. A problem with this approach is it requires energy drawn from the vehicle battery and it is possible to reduce the state of charge of the battery below a minimum threshold level required for starting the vehicle. Accordingly, it will be appreciated that it would be highly desirable to efficiently purge moisture without reducing the state of charge of the vehicle battery below the minimum threshold required for starting the vehicle.